In the handling of crude oil pumped from offshore oil wells, a number of offshore terminals have been developed over the years for use in transferring the production from such wells to tankers at berth or to other receptacles or vessels. Among these are the following: the artificial offshore island made up of steel construction or precast concrete pilings fixed to the sea bottom and supporting steel, precast or cast-in-place concrete structures; the multiple-buoys-mooring system consisting of several mooring buoys anchored around a tanker berth; the tower mooring system consisting of a steel structure fixed to the sea bottom by piles and having a turntable fitted on the top of the structure from which a mooring rope is connected to a tanker at berth adjacent thereto; and the single point mooring system which can be of one of two types, namely, a catenary-chain type, single buoy mooring system or the single anchor leg mooring unit, both of which allow a tanker to rotate freely and to take the position of least resistance to combined external forces, such as those due to waves, sea currents, wind and other types of rotating moorings. All of these various systems can only be compared by selecting a particular site where such systems may be installed. At any such site, water depth and sea and weather conditions are to be considered along with the mooring forces between each system and a vessel to be associated therewith.
All of the foregoing systems have certain drawbacks, especially when the same are used in regions of the sea which have extreme wave action at certain times of the year. For instance, at certain locations in the North Sea where much offshore drilling now takes place, a 100-year wave is estimated to have a height of 95 to 105 feet but such estimates have lately been thought to be erroneous in that a more correct estimate is 150 feet. Under such extreme wave conditions, it is virtually impossible to maintain a mooring between a vessel and any one of the aforesaid offshore terminals without causing damage to both the vessel and the terminal. Also, to disengage and re-engage the vessel relative to the terminal in such extreme wave conditions, special structures and time consuming procedures are required.
The above-mentioned offshore terminals also have other drawbacks including restrictive water depth and excessive costs of construction and maintenance due to their complexity of structural detail. For instance, the artificial offshore island is so costly that its use seems justified only when the oil production handled thereby or throughput is very high, such as above 200,000 barrels per day, and only when very large vessels are available for use with it. Also, the waters around the island should be well protected; otherwise, a breakwater has to be constructed at high cost. Furthermore, the depth at which the aforesaid systems are operable do not exceed about 300 feet.
The multiple-buoys-mooring system finds it application only in protected shallow waters or exposed locations with mild wave climate and for relatively small tankers. It is otherwise too easily damaged; thus, it cannot withstand the extreme weather conditions mentioned above which occur in the North Sea.
The tower mooring system is costly to erect and to operate and presents collision risks. It is believed to find its best application only in relatively shallow waters which are protected from extreme wave action; thus, it is not suitable at all for the open North Sea operations discussed above.
The single point mooring system is not suitable for use under extreme wave conditions because, in the case of the single buoy mooring system, the hawsers which anchor the tanker or vessel break due to impact tension caused by wave forces exerted on the system. These waves also cause the buoy to separate from the vessel and then to slam into it to cause damage to either or both of them. In the case of the single anchor leg mooring system, an anchor chain connects a buoy body to a generally rigid vertical riser standing upwardly from the sea bottom but this chain also breaks due to extreme wave action and separates the buoy from the riser, causing sufficient damage to the system to requires replacement of the buoy, all of which requires a long shutdown time for repairs.
In view of the problems associated with the offshore oil terminals mentioned above, a need has arisen for an improved offshore terminal which is simple and rugged in construction, is relatively inexpensive to produce and maintain, can be easily moved to other locations, can be used to keep a vessel coupled thereto even under extreme wave conditions yet the vessel can safely engage with and disengage from the terminal without special assistance or procedures, and can operate at depths of up to 2,000 feet or more.